1. Field
The present invention relates generally to data communication, and more specifically to techniques for time-sharing a common channelization code among a number of terminals in compressed mode to more efficiently utilize the available channelization codes.
2. Background
Wireless communication systems are widely deployed to provide various types of communication including voice and packet data services. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other multiple access technique. CDMA systems may provide certain advantages over other types of system, including increased system capacity. A CDMA system is typically designed to conform to one or more standards, such as IS-95, cdma2000, and W-CDMA standards, all of which are known in the art and incorporated herein by reference.
The W-CDMA standard supports a “compressed mode” of operation on the downlink whereby data is transmitted from a base station to a terminal within a shortened time duration (i.e., compressed in time). The compressed mode is used in W-CDMA to allow a terminal in active communication with the system (i.e., on a traffic channel) to temporarily leave the system in order to perform measurements on a different frequency and/or a different Radio Access Technology (RAT) without losing data from the system. In the compressed mode, data is transmitted to the terminal during only a portion of a (10 msec) frame so that the remaining portion of the frame (referred to as a transmission gap) may be used by the terminal to perform the measurements.
In accordance with the W-CDMA standard, the reduction in the transmission time for a given compressed frame can be achieved by either (1) reducing the amount of data to transmit to the terminal in the frame, (2) increasing the coding rate, or (3) increasing the data rate. Each of these available mechanisms for transmitting a compressed frame is associated with tradeoffs that may impact performance, as described below.
First, reducing the amount of data to transmit in a compressed frame may be achieved via scheduling at a higher signaling layer, but the reduced amount of data may be undesirable for some applications. For example, for voice users, the reduction in the amount of data may cause a corresponding reduction in the rate of an adaptive multi-rate (AMR) vocoder down to a level (e.g., 5.15 kbps) that may result in significantly reduced quality of service.
Second, increasing the coding rate may be achieved by “puncturing” or deleting some of the coded bits (this mechanism is only available on the downlink). However, as the transmission gap increases (to up to 7 slots out of 15 possible slots in a frame), more coded bits need to be punctured, the coding rate increases, and higher transmit power is needed for the unpunctured bits to achieve the desired level of performance (i.e., a particular bit error rate or frame error rate) at the terminal. To perform inter-frequency/inter-RAT measurements, long transmission gaps (e.g., almost half a frame) will likely be required. However, puncturing is impractical for these longer transmission gaps because of the excessively high transmit power requirement.
And third, increasing the data rate may be achieved by channelizing the data to be transmitted with a shorter-length channelization code. The W-CDMA standard utilizes orthogonal variable spreading factor (OVSF) codes of various lengths to channelize the data prior to transmission to the terminals. The length or spreading factor (SF) of the OVSF codes can range from 4 to 512 chips, with shorter-length codes being capable of supporting correspondingly higher data rates. In accordance with the W-CDMA standard, a compressed frame for a particular terminal may be channelized with a channelization code of half the length (i.e., SF/2) as that of the channelization code used for non-compressed frames for the terminal. Moreover, the W-CDMA standard defines a specific relationship between the channelization code of length SF used for non-compressed frames and the channelization code of length SF/2 used for compressed frames.
The use of the specifically defined channelization code of length SF/2 for compressed frames reduces the number of channelization codes available for other uses by the system. In general, fewer codes are available for use as the spreading factor decreases, e.g., half as many channelization codes are available if the spreading factor is reduced by a factor of two. Conventionally, a channelization code of length SF/2 is assigned to each terminal operating in compressed mode and for the duration in which the terminal is in this mode. Each terminal in compressed mode would then effectively take up two channelization codes of length SF since one channelization code of length SF/2 occupies the “code space” of two channelization codes of length SF. Since W-CDMA may be code-limited on the downlink, assignment of a channelization code of length SF/2 to each terminal for the transmission of compressed frames is undesirable.
There is therefore a need in the art for techniques to more efficiently utilize channelization codes, especially in the compressed mode defined by the W-CDMA standard.